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### Collaborating Authors

### Zielona Góra

### A Laboratory Measurements For Geological Samples (By Pulsed Method)

Czubek, Jan A. (Henryk Niewodniczaalski Institute of Nuclear Physics) | Drozdowicz, Krzysztof (Henryk Niewodniczaalski Institute of Nuclear Physics) | Gabañska, Barbara (Henryk Niewodniczaalski Institute of Nuclear Physics) | Igielski, Andrzej (Henryk Niewodniczaalski Institute of Nuclear Physics) | Krynicka, Ewa (Henryk Niewodniczaalski Institute of Nuclear Physics) | Woznicka, Urszula (Henryk Niewodniczaalski Institute of Nuclear Physics)

ABSTRACT The advances in a method of measurement of the thermal neutron macroscopic absorption cross section ?a of small rock samples are presented. Theoretical principles of the method have been established in a one diffusion group approach, where thermal neutron parameters used have been averaged over the modified Maxwellian. In the consecutive measurements the investigated sample is enveloped in shells of a moderator of varying thickness. The neutron parameters of the moderator should be well known. The entire sample-moderator system is irradiated by a pulsed beam of fast neutrons. The neutrons are slowed down in the system and the die-away rate of escaping thermal neutron flux is measured. The time decay constant versus the thickness of the moderator creates an experimental curve. The absorption cross section of the sample is found from the intersection of the experimental curve with a theoretical one which is calculated for the case, when the dynamic material buckling of the inner sample is set to zero. No reference absorption standard is required and the method is independent of the transport cross section of the measured sample. For geological formations the measured samples are composed from the crushed rock matrix saturated with a highly absorbing liquid. A calculation of the mass absorption cross section of the rock matrix requires the knowledge of the following quantitites: the decay constant of the neutron die-away curve, the neutron parameters of the external moderator (Plexigaiss), and the absorption cross section of the saturating fluid. The sample volume is about 170 ccm. The standard deviation of the measured mass absorption cross section of the rock matrix is in the range of 4 to 20 per cent of the measured value and for brines is of the order of 0.5 per cent. The highest value of the standard deviation arrives for the lowest values of the measured absorption cross section i. e. for pure silica. About one hundred rock samples of different lithology and stratigraphy have been measured until now. The Polish neutron calibration facility (at Zielona Gora) was also sampled by this method. Another faster and cheeper procedure was developed in the frame of this method for its routine applications. This method was established through a careful consideration of all experimental results obtained till now. It requires a single die-away measurement performed with only one size of the Plexiglass moderator, A detailed discussion of the procedure has allowed to apply a computer simulation of the behaviour of the statistical errors to estimate the accuracy for each fast assay of the sample.

ABSTRACT The signal of a neutron porosity tool depends upon the tool, borehole and formation properties. One of the solutions to the problem of calibration of the neutron porosity tools is the correlation of the tool readings with rock neutron properties, such as slowing-down length or migration length, i. e. with the rock properties characterizing it in its homogeneous part undisturbed by the presence of a borehole or tool. This way of calibration to some extent eliminates the influence of the rock lithology but also has a serious handicap because it is valid for a fixed tool-borehole geometry only. The new method of semi empirical neutron tool calibration offers a much wider range of possibilities which by correlation of the tool readings with some apparent migration of slowing down lengths are free of the influence of formation lithology, borehole, tool sizes, etc. The apparent migration or slowing down lengths are calculated as the space parameters characterizing the neutron field inside the borehole and are calculated for real measurement conditions. All borehole and lithology correction factors to the standard porosity calibration curve are easily obtainable using the approach proposed in the paper. Examples of the real calibration curves obtained at the Zielona Gora (Poland) calibration center are shown.